1. Field of the Invention
The present invention relates to integrated circuit output buffers. In particular, the present invention is a temperature compensated output buffer which is adapted for impedance matched coupling to a transmission line at both room temperature and the cryogenic temperature of liquid nitrogen.
2. Description of the Prior Art.
Tri-state output buffers are well known circuit elements commonly found on integrated circuits. These output buffers are used to interface main logic or other signal processing circuitry on the integrated circuit to the device's output pins. When driven to its tri-state mode, an output buffer presents a very high output impedance at the output terminal, thereby electrically isolating the transmission line or output device to which the output terminal is coupled from the integrated circuit. When operated in its normal mode, the logic state of the output buffer will be controlled by the signal processing circuitry which it interfaces to the output pin. The output buffer provides these date signals at speeds and signal levels required by the output devices.
Known output buffers include an output stage formed by several transistors. The output impedance of output buffers of this type is essentially equal to the output impedance of the output stage. This output impedance, in turn, has a relatively large temperature coefficient of resistance since the resistance characteristics of the transistors are very sensitive to the ambient temperature in which they are operated.
It is well known that in order to properly couple an output signal from an output buffer to a transmission line or output device, it is required that the output impedance of the output buffer be equal to the characteristic impedance of the transmission line. While this is not a problem for output buffers which are adapted to operate at one temperature, it does present problems when the integrated circuit must be used at both room temperature, and immersed in liquid nitrogen as are some integrated circuits used in supercomputers. If the transistors forming the output stage are sized to have an output impedance which matches the transmission line when used at room temperature, the impedance of the output stage will drop drastically when immersed in liquid nitrogen. The result is an unacceptable impedance match between the output buffer and transmission line.
Clearly, there is a continuing need for improved output buffers. In particular, there is a need for an integrated circuit output buffer which can be impedance matched to a transmission line at both room temperature and the temperature of liquid nitrogen. This output buffer must of course be relatively simple and inexpensive to manufacture.